Structure analysis of the 6H-Ba(Ti,Fe3+,Fe4+)O3-δ solid solution

The 6H form of BaTiO3 forms an extensive solid solution with barium iron oxide, having the general formula BaTi1-x-yFex3+ Fey4+O3-x/2, where the Fe3+/Fe4+ ratio depends on reaction temperature and gas atmosphere. Two series of solid solutions with iron predominantly in the form Fe3+ or Fe4+ were prepared by quenching from high temperatures in air or slow cooling to 200 degrees C in oxygen, respectively. The structural variations in both series were obtained from Rietveld refinements of powder X-ray diffraction data, For the series with y similar to 0, the progressive substitution of Ti4+ by Fe3+ is accompanied by the formation of O(1) oxygen vacancies in the h-BaO3 layers that separate pairs of occupied face-shared octahedra, The maximum vacancy concentration occurs at x = 0.67 which corresponds to a composition of Ba square O-2 (square = vacancy) in the h-stacked layers. Solid solution members with x > 0.67 were unstable relative to other polymorphs Further incorporation of iron into the 6H structure at high temperature occurs by substitution of Ti4+ by Fe4+, i.e., by increasing y. The observed structural variations in the solid solution phases are explained in terms of the Ba and Fe, Ti atom packing and the changes in interatomic interactions between the metal atoms as oxygen is removed from the O(1) site. (C) 1998 Academic Press.